Rodents use whiskers as their primary tactile sensors. Individual whiskers are innervated by a group of touch sensory neurons located in the trigeminal ganglion, and are mapped onto the brain by the central axons from the same set of trigeminal neurons. The discovery of the whisker- related large modular structures (barrelettes in the hindbrain, barreloids in the thalamus and barrels in the cortex) in 1970s made the trigeminal-whisker system a popular prototypic model for studying somatosensory maps and touch sensory circuits. Yet to date, we still have limited knowledge of the precise neural circuits assembled inside each whisker-representing unit. The objective of this proposal is to dissect the micro-circuits formed by different classes of touch sensory neurons within individual barrelettes, and to begin to identify the molecular mechanisms controlling the precise assembly of these barrelette circuits. These studies will provide the much- needed knowledge for future understanding of somatosensory information coding and processing and ultimately sensory perception. The mechanisms uncovered in this study will also have broad implications for understanding neuropathological diseases associated with abnormal wiring in the somatosensory system such as Autism and neuropathic orofacial pain.